Majusculamide C

ABSTRACT

Majusculamide C, a novel cyclic peptide compound which inhibits fungal plant pathogens, process for its preparation from a deep-water blue-green alga, Lyngbya majuscula, and process for inhibiting fungal plant pathogens with and fungicidal compositions containing majusculamide C.

The Government has rights in this invention pursuant to Grant No. CHE 79-25416 awarded by the National Science Foundation.

SUMMARY OF THE INVENTION

This invention relates to a new cyclic peptide which is designated majusculamide C. We have found that majusculamide C is contained in marine organisms, in particular in an algal association, the primary constituent of which is the blue-green alga Lyngbya majuscula. This algal association can be collected from Pinnacles in Enewetak lagoon at Enewetak atoll, which is located in the Marshall Islands.

Accordingly, this invention relates in one aspect to the isolation of majusculamide C from natural sources, especially marine organisms, particularly a deep-water strain of blue-green alga of the species Lyngbya majuscula. Majusculamide C in substantially pure form, i.e., free from naturally occurring byproducts, is a further feature of the present invention. In other aspects, this invention relates to the use of majusculamide C to inhibit fungal plant pathogens and to fungicidal compositions containing majusculamide C.

DESCRIPTION OF THE DRAWING

The infrared spectrum of majusculamide C, run in KBr disc, is provided in the accompanying drawing.

DETAILED DESCRIPTION OF THE INVENTION

Fungal plant pathogens cause great economic losses each year. New agents effective against these phytopathogens are needed. Presently used fungicides differ in their effectiveness against specific fungi. In addition, resistant fungal strains frequently develop, creating a continual need for new, effective agents.

This invention provides a new cyclic peptide compound which is designated majusculamide C. Majusculamide C is useful against a number of fungal plant pathogens such as, for example, tomato late blight and grape downy mildew.

Majusculamide C has thus far been found in an algal association, the primary constituent of which is a deep-water seaweed of the family Oscillatoriaceae, the blue-green alga Lyngbya majuscula. The term "deep water" used herein refers to depths of from about 25 to about 100 feet. This term is used to distinguish the algal association of this invention from Lyngbya majuscula found in the shallow waters, i.e. depths of less than about 20 feet, of Enewetak lagoon. The shallow-water Lyngbya found there do not contain majusculamide C.

The alga of this invention is found in the lagoon at Enewetak atoll in the Marshall Islands. In addition to majusculamide C, the alga produces a toxin, debromoaplysiatoxin, which causes a severe contact dermatitis called "swimmer's itch" [see Mynderse et al., Science 196, 538-540 (1977)]. This toxin is separated from majusculamide C by chromatography.

Majusculamides A and B are major lipophilic constituents of several shallow-water varieties of Lyngbya majuscula found in Hawaii and Enewetak [see Marner et al., J. Org. Chem. 42, 2815-2819 (1977)]. Majusculamide B is N-[(S)-2-methyl-3-oxodecanoyl]-D-N,O-dimethyltyrosyl-L-N-methylvalinamide. Majusculamide A is the (2R)-2-methyl-3-oxodecanoyl epimer of majusculamide B. Majusculamides A and B were not found in the deep-water L. majuscula which contained majusculamide C.

Majusculamide C can be isolated from the natural source, e.g., from the blue-green alga Lyngbya majuscula, using techniques which will be apparent to those skilled in the art. For example, a preferred method of isolating majusculamide C is to extract the freeze-dried alga exhaustively with hexane, acetone, and methanol. The organic solvents are then combined and evaporated to give an extract. The extract is partitioned between hexane and 90% aqueous methanol. The methanolic portion is then partitioned between dichloromethane and 80% aqueous methanol. The dichloromethane portion is chromatographed over Sephadex (LH-20) to give majusculamide C which can be further purified by high performance liquid chromatography (HPLC).

Majusculamide C is a white amorphous solid which has a molecular weight of approximately 984, based on field-desorption mass spectral data.

Majusculamide C is soluble in solvents such as methanol, dichloromethane, and acetone but is not soluble in water. The specific optical rotation, [α]_(D), of majusculamide C is -96° (c 2.5, CH₂ Cl₂).

The infrared (IR) spectrum of majusculamide C in KBr disc is shown in the accompanying drawing. The IR spectrum has the following absorption maxima:

    ______________________________________                                         Wavenumber (cm.sup.-1)                                                                         Percent Transmittance                                          ______________________________________                                         3336.13         46.73                                                          2967.70         38.329                                                         2937.80         46.061                                                         2878.97         53.83                                                          2837.49         68.28                                                          1741.85         40.857                                                         1680.12         15.247                                                         1643.47         9.872                                                          1585.60         56.29                                                          1514.23         16.761                                                         1465.04         34.352                                                         1411.03         43.91                                                          1385.95         47.18                                                          1373.42         49.82                                                          1357.98         52.62                                                          1301.08         44.04                                                          1289.51         43.16                                                          1249.00         36.099                                                         1180.52         43.466                                                         1142.91         48.76                                                          1127.47         47.32                                                          1109.15         50.70                                                          1086.93         51.31                                                          1033.92         49.04                                                          1000.16         57.66                                                          946.15          64.56                                                          908.54          70.48                                                          857.42          69.30                                                          841.99          64.20                                                          827.52          64.63                                                          812.09          66.6                                                           783.16          67.19                                                          ______________________________________                                    

The ultraviolet absorption spectrum of majusculamide C in ethanol shows absorption maxima at 278 nm (ε 1420) and 230 nm (ε 5900).

¹ H and ¹³ C nuclear magnetic resonance (NMR) spectral and mass spectral analyses indicate that the molecular composition of majusculamide C is C₅₀ H₈₀ N₈ O₁₂. Nuclear magnetic resonance analysis also shows that two glycyl units, one alanyl unit, one N-methylvalyl unit, and one N,O-dimethyltyrosyl unit are present. Acid hydrolysis (1.5 N HCl, 25% ethanol-water, reflux 24 hours) of majusculamide C leads to L-alanine, glycine, L-N-methylvaline, L-N-O-dimethyltyrosine, L-N-methylisoleucine, 2-hydroxy-3-methylpentanoylglycine, 2-amino-3-oxo-4-methylpentane (isolated as the hydrochloride), and 3-amino-2-methylpentanoic acid. Thus, the following eight sub-units (1a through 1h) account for all the atoms in the molecular formula of majusculamide C: ##STR1##

Table 1 gives the ¹ H NMR data for majusculamide C at 360 MHz.

                  TABLE 1                                                          ______________________________________                                         .sup.1 H NMR Signals of Majusculamide C                                        Chemical Shift*                                                                               Multiplicity**                                                  ______________________________________                                         7.767                 d,     J = 8.1 Hz                                        7.573                 d,     J = 7.4 Hz                                        7.355                 t,     J = 5.4 Hz                                        7.309                 d,     J = 6.7 Hz                                        7.129          2H,    d,     J = 8.5 Hz                                        7.068                 d,     J = 10.2 Hz                                       6.834          2H     d,     J = 8.5 Hz                                        5.186                 d,     J = 3.2 Hz                                        5.132                 t,     J = 7.3 Hz                                        4.893                 p,     J = 6.8 Hz                                        4.888                 d,     J = 11.1 Hz                                       4.777                 d,     J = 10.7 Hz                                       4.606                 dd,    J = 8.3, 17.7 Hz                                  4.509                 tdd,   J = 10.2, 5.7, 2.7 Hz                             4.437                 dd,    J = 15.9, 5.4 Hz                                  4.401                 dd,    J = 2.1, 6.7 Hz                                   3.738          3H,    s                                                        3.541                 dd,    J = 15.9, 5.0 Hz                                  3.445                 dd,    J = 17.6, 1.0 Hz                                  3.237                 dd,    J = 14.2, 7.0 Hz                                  3.201          3H,    s                                                        2.944          3H,    s                                                        2.938          3H,    s                                                        2.792                 dd,    J = 8.1, 14.0 Hz                                  2.735                 dq,    J = 2.6, 7.0 Hz                                   2.250                 ds,    J = 10.7, 6.7 Hz                                  2.206          2H,    m                                                        1.508          3H,    s                                                        1.2-1.6        6H,    m                                                        1.462          3H,    s                                                        1.136          3H,    d,     J = 6.8 Hz                                        1.082          3H,    d,     J = 7.0 Hz                                        1.042          3H,    d,     J = 6.6 Hz                                        1.024          3H,    d,     J = 6.2 Hz                                        0.916          3H,    d,     J = 7.4 Hz                                        0.873          3H,    d,     J = 7.5 Hz                                        0.862          6H,    t,     J = 7.7 Hz                                        0.738          3H,    d,     J = 6.6 Hz                                        0.393          3H,    d,     J = 6.5 Hz                                        ______________________________________                                          *Chemical shifts are the values in chloroformd in ppm.                         A signal represents one proton unless otherwise described.                     **s = singlet, d = doublet, dd = doublet of doublets, tdd = triplet of         doublets of doublets, dq = doublet of quartets, ds = doublet of septets,       = pentet.                                                                

The ¹³ C NMR data for majusculamide C are listed in Table 2.

                  TABLE 2                                                          ______________________________________                                         .sup.13 C NMR Signals of Majusculamide C in Deuteriochloroform*                ______________________________________                                         209.97(s)                                                                               158.58(s)                                                                               55.06(s)   32.63(d)                                                                              18.95(q)                                   172.64(s)                                                                               130.28(d)                                                                               54.72(s)   30.17(s)                                                                              18.32(q)                                   172.46(s)                                                                               130.28(d)                                                                               51.45(d)   29.08(s)                                                                              18.27(q)                                   171.90(s)                                                                               128.55(s)                                                                               51.07(d)   29.04(s)                                                                              15.31(q)                                   170.91(s)                                                                               114.15(s)                                                                               48.15(d)   26.86(d)                                                                              15.30(q)                                   170.12(s)                                                                               114.15(s)                                                                               42.36(d)   25.85(t)                                                                              15.30(q)                                   169.99(s)                                                                                78.20(d)                                                                               40.62(t)   24.80(t)                                                                              11.48(q)                                   169.86(s)                                                                                60.98(d)                                                                               40.45(t)   23.62(t)                                                                              10.80(q)                                   169.21(s)                                                                                60.86(d)                                                                               37.20(d)   21.98(q)                                                                               9.88(q)                                   167.85(s)                                                                                57.98(d)                                                                               34.46(t)   21.30(q)                                                                               9.60(q)                                   ______________________________________                                          *Chemical shifts are expressed in ppm. The multiplicity is in parentheses                                                                               

Table 3 lists conclusions which can be drawn about the structure of majusculamide C based on the ¹ H NMR and ¹³ C NMR spectra.

                  TABLE 3                                                          ______________________________________                                         Structural Conclusions for Majusculamide C from                                .sup.1 H NMR and .sup.13 C NMR Spectra                                         Chemical Shift   Number and Kinds of Carbons                                   ______________________________________                                         209.97           1 Ketone carbonyl                                             167.85, 169.21, 169.86,                                                        169.99, 170.12, 170.91,                                                                         9 Amide/ester carbonyls                                       171.90, 172.46, 172.64                                                         128.55, 158.58   2 Aromatic tertiary carbons                                   114.15(2), 130.28(2)                                                                            4 Aromatic methine carbons                                    55.06            1 Methoxy                                                     29.04, 29.08, 30.17                                                                             3 N-methyls                                                   54.72            1 Tertiary carbon                                             26.86, 32.63, 37.20, 42.36,                                                    48.15, 51.07, 51.45, 57.98,                                                                     11 Methines                                                   60.86, 60.98, 78.20                                                            23.62, 24.80, 25.85, 34.46,                                                    40.45, 40.62     6 Methylenes                                                  9.60, 9.88, 10.80, 11.48,                                                      15.30, 15.30, 15.31, 18.27                                                                      12 Methyls                                                    18.32, 18.95, 21.30, 21.98                                                     ______________________________________                                    

Electron-impact mass spectrometry (low resolution) analysis of majusculamide C gives the following fragmentation pattern, m/z (rel intensity): 757(5), 756(13), 597(14), 596(38), 566(21), 565(62), 508(2), 452(6), 396(19), 395(80), 381(2), 324(2), 275(9), 248(5), 211(6), 204(28), 164(12), 161(19), 155(12), 121(13), 114(100%).

The observed and calculated fragments of majusculamide C on high resolution electron impact mass spectroscopy are summarized in Table 4.

                  TABLE 4                                                          ______________________________________                                         High Resolution Mass Spectral Data of                                          Majusculamide C                                                                Calculated           Observed                                                  ______________________________________                                         C.sub.12 H.sub.19 O.sub.3                                                                      211.13342                                                                               211.1327                                              C.sub.17 H.sub.26 O.sub.5 N                                                                    324.18108                                                                               324.1786                                              C.sub.21 H.sub.35 O.sub.5 N.sub.2                                                              395.25458                                                                               395.2529                                              C.sub.23 H.sub.38 O.sub.6 N.sub.3                                                              452.27606                                                                               452.2730                                              C.sub.29 H.sub.49 O.sub.7 N.sub.4                                                              565.36013                                                                               565.3570                                              C.sub.40 H.sub.62 O.sub.9 N.sub.5                                                              756.45476                                                                               756.4455                                              ______________________________________                                    

Sequencing based on the mass spectral data suggests that a glycyl-N-methylisoleucylglycyl-N-methylvalyl-N,O-dimethyltyrosyl sequence is present in majusculamide C.

Majusculamide C has a retention time of approximately 14.7 minutes on high performance liquid chromatography (HPLC), using a Dupont Zorbax C-8 4.6-mm×25-cm column, an acetonitrile:water (15:85) mobile phase, and a flow rate of 2 ml per minute.

Based on the characteristics known thus far, the structure shown in formula 2 has been postulated as the tentative structure for majusculamide C: ##STR2##

This invention also relates to a method of protecting plants from phytopathogenic fungi which comprises contacting the loci of the fungi with a fungicidally-effective amount of majusculamide C. The loci of the fungi can be a portion of the plant, such as leaves, stems, flowers or roots, or the soil wherein the fungi may be located. Majusculamide C appears to translocate from roots to shoots; consequently it is possible that application could be made to seeds, roots, etc. to obtain fungicidal effect throughout the plant. Application rates will vary according to a number of factors, such as the location of the plants being protected and the severity of the fungal infection. Thus, for use in a greenhouse, the fungicidal compound is applied as a soil drench using a composition having a concentration in the range of from about 1 to about 200 ppm of active ingredient, preferably from about 5 to about 100 ppm. As is understood by those in the art, application rates used in the field are usually greater than those used in a greenhouse, and range from about 25 to about 1000 ppm.

Majusculamide C has been shown by suitable tests to control a number of fungi, including Phytophthora infestans, the causative organism of tomato late blight, Plasmopora viticola, the causative organism of grape downy mildew and Rhizoctonia solani, the causative organism of Rhizoctonia damping-off.

In another embodiment, this invention relates to compositions suitable for inhibiting plant-pathogenic fungi comprising (1) majusculamide C in an amount effective to inhibit the growth of a plant-pathogenic fungus and (2) a suitable carrier.

The compositions for use in this embodiment desirably contain, in addition to the majusculamide C, one or more of a plurality of additaments including water, polyhydroxy compounds, petroleum distillates, and other dispersion media, surface-active dispersing agents, emulsifiers, and finely-divided inert solids. The concentration of majusculamide C in these compositions will vary depending on whether the composition is intended for direct application to plants or is intended to be subsequently diluted with additional inert carrier such as water to produce the ultimate treating composition.

Treating compositions are most conveniently formulated by preparing liquid or solid concentrates, which are subsequently diluted to the desired level for use. Emulsifiable liquid concentrates can be prepared by incorporating from about 1 to about 10 percent by weight of the active ingredient and an emulsifiable agent in a suitable water-immiscible organic liquid. Such concentrates may be further diluted with water to form spray mixtures in the form of oil-in-water emulsions. Such spray compositions then comprise active compound, water-immiscible solvent, emulsifying agent, and water. Suitable emulsifying agents can be of the nonionic or ionic types, or blends thereof, and include condensation products of alkylene oxides with phenols and organic acids, polyoxyethylene derivatives of sorbitan esters, complex ether alcohols, ionics of the arylalkyl sulfonate type, and the like. Suitable water-immiscible organic liquids include aromatic hydrocarbons, aliphatic hydrocarbons, cycloaliphatic hydrocarbons, and mixtures thereof such as petroleum distillates.

Solid concentrate mixtures can be prepared by incorporating from about 10 to about 50% by weight of active compound in a finely-divided solid carrier such as bentonite, Fuller's earth, diatomaceous earth, hydrated silica, diatomaceous silica, expanded mica, talc, chalk, and the like. Such concentrates can be formulated, if desired, for direct use as dusting compositions, or can be diluted, if desired, with additional inert solid carriers to produce dusting powders containing around 0.05 to 1% by weight of majusculamide C. Alternatively, the surfactants, that is, dispersing and/or wetting agents, can be incorporated along with majusculamide C in the solid carrier to form wettable powder concentrates ranging from about 10 to about 25% by weight concentration, which subsequently can be dispersed in water or other hydroxylated carrier to form spray compositions. Suitable surfactants include condensed aryl sulfonic acids and sodium salts thereof, sodium lignosulfate, sulfonate-oxide condensate blends, alkylaryl polyether alcohols, sulfonate/nonionic blends, anionic wetting agents, and the like.

Further, majusculamide C can be incorporated in solutions, simple dispersions, aerosol formulations, and other media acceptable for treating vegetation or for applying to the soil.

The antifungal composition of this embodiment is applied to infected or susceptible plant or soil surfaces in any convenient fashion such as by spraying, dusting, dipping, or drenching. A spray method is considered preferable, especially when large numbers of plants are involved, because such a treatment is faster and more uniform. In spraying it is usually sufficient for the infected or susceptible surfaces to be made thoroughly wet with the liquid dispersion. Good results can be obtained by using spray compositions whether they are emulsions or aqueous dispersions of solid concentrates.

Where the fungi to be controlled are in the soil, the antifungal compound can be applied to the soil directly, or it can be diluted with various inert solid or liquid diluents and then applied to the soil. In one method of application the soil surface is sprayed with a liquid dispersion or emulsion of the active ingredient. The application is allowed to remain as a coating on the surface of the soil or, alternatively, is incorporated into the soil by disking, hoeing, or other methods known to those in the art. Another method of application is to apply the active ingredient in the form of a liquid dispersion or emulsion to the soil as a drench. Thus, for the control of soil-inhabiting fungi in the greenhouse, the application rate varies from about 5 to about 200 ppm active ingredient.

Majusculamide C can also be used as a seed soak for seeds prior to planting. A suitable seed soak formulation contains majusculamide C together with excipients such as a mixture of ethanol-acetone, polyoxyethylene sorbitan monolaurate, and the like.

When used as a seed soak, control can be accomplished at an application rate of from about 50 to about 400 ppm of majusculamide C. The seeds are allowed to soak in the formulation for about 4 hours and then are removed and planted.

The activity of majusculamide C against plant pathogenic fungi in standard in vitro agar-dilution tests is summarized in Tables 5-10. In these tests 100× solutions of majusculamide C in 95% aqueous ethanol were used. Majusculamide C solution (0.1 ml) and melted agar (9.9 ml) at 50° C. were mixed. An inoculum plug (6-mm diameter) was placed in the center of each plate; the radial growth of the colony was measured. Zone of growth was measured from the edge of the plug to the edge of the colony.

                  TABLE 5                                                          ______________________________________                                         In Vitro Agar Dilution Activity of Majusculamide C                                                          Pythium                                                  Rhizoctonia                                                                              Sclerotinia aphani-                                                  solani    homoeocarpa dermatum                                                            Per-          Per-        Per-                                                 cent          cent        cent                               Majuscul-         In-           In-         In-                                amide C  Zone of  hibi-  Zone of                                                                               hibi-                                                                               Zone of                                                                               hibi-                              Level (ppm)                                                                             Growth   tion   Growth tion Growth tion                               ______________________________________                                         1        19       30     17     26   1       97                                10       10       63     16     30   0      100                                 0.sup.a 27        0     23      0   28      0                                 ______________________________________                                          .sup.a Control                                                           

The ED₅₀ (effective dose to achieve 50% inhibition) values for majusculamide C against the organisms in Table 5, calculated using the results given there, are summarized in Table 6.

                  TABLE 6                                                          ______________________________________                                         ED.sub.50 Values for Majusculamide C                                           Organism          ED.sub.50                                                    ______________________________________                                         Rhizoctonia solani                                                                                4 ppm                                                       Sclerotinia homoeocarpa                                                                          >10 ppm                                                      Pythium aphanidermatum                                                                           <1 ppm                                                       ______________________________________                                    

                                      TABLE 7                                      __________________________________________________________________________     In Vitro Agar-Dilution Activity of Majusculamide C and Ridomil                              Pythia    Aphanomyces                                                                              Phytophthora                                               aphanidermatum                                                                           euteiches infestans                                              Level                                                                              Zone of                                                                             Percent                                                                             Zone of                                                                             Percent                                                                             Zone of                                                                             Percent                                  Compound (ppm)                                                                              Growth                                                                              Inhibition                                                                          Growth                                                                              Inhibition                                                                          Growth                                                                              Inhibition                               __________________________________________________________________________     Majusculamide C                                                                         0.01                                                                               30   6    27   7    10   17                                         "      0.1 12   62   27   7    7    42                                         "      1.0 1.5  95   20.5 29   6.5  46                                         "      10.0                                                                               0    100  3.5  88   0    100                                      Ridomil (tech.)                                                                         0.1 21   34   27   7    6    50                                         "      1.0 2    94   28   3    2    83                                       Control  0   32   0    29   0    12   0                                        __________________________________________________________________________

The ED₅₀ values for majusculamide C against the organisms in Table 7, calculated using the results given there, are summarized in Table 8.

                  TABLE 8                                                          ______________________________________                                         ED.sub.50 Values for Majusculamide C                                           Organism           ED.sub.50                                                   ______________________________________                                         Pythium aphanidermatum                                                                            0.07 ppm                                                    Aphanomyces euteiches                                                                             2.1 ppm                                                     Phytophthora infestans                                                                            1.07 ppm                                                    ______________________________________                                    

In Table 9 the activity of majusculamide C against two Ridomil-sensitive and two Ridomil-resistant strains is compared.

                                      TABLE 9                                      __________________________________________________________________________     In Vitro Activity of Majusculamide C Against Ridomil-Resistant Strains                  Pythium ultimum     Phytophthora capsici                                       Ridomil-sensitive                                                                        Ridomil-resistant                                                                        Ridomil-Sensitive                                                                        Ridomil-resistant                                strain    strain.sup.a                                                                             strain    strain.sup.a                            Majusculamide C                                                                         Zone of                                                                             Percent                                                                             Zone of                                                                             Percent                                                                             Zone of                                                                             Percent                                                                             Zone of                                                                             Percent                            Level (ppm)                                                                             Growth                                                                              Inhibition                                                                          Growth                                                                              Inhibition                                                                          Growth                                                                              Inhibition                                                                          Growth                                                                              Inhibition                         __________________________________________________________________________     0.1      18.5 53   15.5 60   14.5 46   14.5 48                                 1        4.5  88   3.5  91   7    74   4.5  84                                 10       0    100  0.sup.b                                                                             100  0    100  0.5  98                                 0.sup.c  39   0    39   0    27   0    28   0                                  __________________________________________________________________________      .sup.a Maintained on V8 agar with 100 ppm Ridomil                              .sup.b No growth on plug                                                       .sup.c Control                                                           

The ED₅₀ values for majusculamide C against the organisms in Table 9, calculated using the results given there, are summarized in Table 10.

                  TABLE 10                                                         ______________________________________                                         ED.sub.50 Values for Majusculamide C                                           Against Ridomil-Resistant Strains                                              Organism         ED.sub.50 (ppm)                                               ______________________________________                                         Pythium ultimum                                                                (Ridomil-sensitive)                                                                             <0.1                                                          Pythium ultimum                                                                (Ridomil-resistant)                                                                             <0.5                                                          Phytophthora capsici                                                           (Ridomil-sensitive)                                                                             0.45                                                          Phytophthora capsici                                                           (Ridomil-resistant)                                                                             0.25                                                          ______________________________________                                    

Majusculamide C was effective when tested in vivo against Phytophthora infestans, the causative agent of tomato late blight, and Plasmopora viticola, the causative agent of grape downy mildew. In these tests majusculamide C was formulated in a solvent of acetone:ethanol (1:1) plus 0.05% Tween 20. Tomato leaves (C. V. Rutgers) were placed, top side up, on rubber mats in Petri dishes. Grape leaves (C. V. Niagara) were placed, bottom side up, on filter paper on rubber mats in Petri dishes. Foliar spray solutions of test compounds were placed on the leaves, using two replicates per dosage per leaf type. Leaves were dried, and water (approximately 10 ml) was added to the bottom of the Petri dish. Tomato leaves were inoculated with a zoospore suspension of P. infestans. Grape leaves were inoculated with a sporangial suspension of P. viticola. Tomato leaves were incubated in a 15° C. incubator for 48 hr and then at room temperature for 5 days. Grape leaves were incubated in a lighted chamber at 20° C. for 7 days.

Leaves were rated on a 1-5 scale wherein 1=severe disease and 5=no disease. Phytotoxicity was also rated on a 1-5 scale with 0=no injury and 5=severe injury.

Table 11 summarizes the results of these tests.

                  TABLE 11.                                                        ______________________________________                                         In Vivo Activity of Majusculamide C                                                       Disease Ratings.sup.a                                                          Tomato      Grape                                                              Late Blight Downy Mildew                                            Majusculamide C                                                                             Disease  Phyto-   Disease                                                                               Phyto-                                   Dosage (ppm) Control  toxicity Control                                                                               toxicity                                 ______________________________________                                         400          5        4.sup.b   4+    0                                        100          5        3.5.sup.b                                                                                4-    0                                        25           5        0        3      0                                        6.25         5        0        2      0                                        0.sup.c      1        0        1      0                                        ______________________________________                                          .sup.a Average of two replicates                                               .sup.b Evidence of chlorosis                                                   .sup.c Control                                                           

In order to illustrate this invention more fully, the following examples are provided.

EXAMPLE 1

The starting material is a blue-green alga species, Lyngbya majuscula, which can be collected in the lagoon on Enewetak atoll. The L. majuscula used in this Example was collected from Reefer 8 Pinnacle, Enewetak lagoon, at depths of from about 35 to about 60 feet. The alga is reddish in color and grows in hairlike strands. The alga is rinsed with sea water to eliminate major contaminating organisms.

Frozen L. majuscula (3 kg wet weight) thus collected was homogenized and extracted with a mixture of chloroform and methanol (1:2 by volume). Water was added to the filtrate; the chloroform layer was washed repeatedly with water, then was dried over anhydrous sodium sulfate and evaporated to give a crude extract (22 g). This extract (dissolved in chloroform) was chromatographed over a 4.7-×40-cm Florisil column (<200 mesh).

The column was developed with hexane (400 ml), hexane:chloroform mixtures (9:1-200 ml; 3:1-300 ml; 1:1-400 ml), chloroform (400 ml) and chloroform: methanol mixtures (9:1-400 ml; 3:1-400 ml; 1:1-200 ml). The following fractions were collected:

    ______________________________________                                         Fraction No.   Volume (ml)                                                     ______________________________________                                         0              250                                                             1              300                                                             2              150                                                             3              300                                                             4              250                                                             5              250                                                             6              225                                                             7              250                                                             8              250                                                             9              600                                                             10             250                                                             ______________________________________                                    

Fractions 5 and 6, collected using hexane:chloroform (1:1) and chloroform, provided, after evaporation, 2.14 g and 1.94 g of substantially pure majusculamide C, respectively.

Subsequent development of the Florisil column, using chloroform:methanol (9:1), gave crude debromoaplysiatoxin which was purified over a silica gel column to give 400 mg of pure debromoaplysiatoxin.

Chromatography of fraction #5 from the Florisil column over a 1.5-cm×1.15-m column of Sephadex LH-20, eluting with chloroform:methanol (1:1) and collecting fractions based on proton magnetic resonance data, gave first fatty compounds and chlorophyll and then gave majusculamide C in an especially pure form (1.69 g).

EXAMPLE 2

The L. majuscula used in this experiment was collected at Enewetak from the South Medren Pinnacle in Enewetak lagoon in depths ranging from 35 to 60 feet. Freeze-dried L. majuscula (748 g) was exhaustively extracted with hexane, acetone, and methanol, respectively; the extracts were combined and dried to give 19 g of extract. This crude extract was partitioned between hexane (500 ml) and 90% methanol (100 ml). The resulting methanol layer was diluted with water and then was extracted with dichloromethane (500 ml). The dichloromethane was evaporated to give 4.8 g of brown semisolid which represented 25% of the crude extract, or 0.6% of the dried seaweed.

Majusculamide C was isolated from the dichloromethane extract by gel filtration. A portion of the dichloromethane extract (1.5 to 2 g) was filtered through a 180-×2.5-cm LH-20 Sephadex column, using CH₂ Cl₂ :CH₃ OH (1:1) as the solvent system and collecting 50-ml fractions. Crude majusculamide C was eluted in the third fraction.

Majusculamide C was further purified by HPLC with a Whatman Partisil ODS-2 reverse phase column. Purification was achieved using 30 mg of crude majusculamide C, a CH₃ CN:H₂ O (7:3) mobile phase, a pressure of 300 psi and a flow rate of 1 ml per minute. Majusculamide C was retained for about 100 to 110 minutes. The eluant yielded a clear amorphous solid upon evaporation. 

We claim:
 1. Majusculamide C, which is a white amorphous solid having these characteristics:(a) a molecular weight of approximately 984; (b) an empirical formula of about C₅₀ H₈₀ N₈ O₁₂ ; (c) a specific optical rotation, [α]_(D), of -96° (c 2.5, CH₂ Cl₂); (d) an infrared spectrum as shown in the accompanying drawing; (e) an ultraviolet absorption spectrum in ethanol with absorption maxima at 278 nm (ε 1420) and 230 nm (ε 5900); (f) is soluble in solvents such as methanol, dichloromethane, and acetone, but is insoluble in water; (g) contains the structural sub-units 1a-1h: ##STR3## (h) appears to contain the sub-unit grouping: glycyl-N-methylisoleucylglycyl-N-methylvalyl-N,O-dimethyltyrosyl; and (i) the following tentative structure: ##STR4##
 2. A process to produce the compound of claim 1 which comprises treating a deep-water algal association containing as its primary constituent the blue-green alga Lyngbya majuscula, which process comprises extracting the alga with an organic solvent in which the compound is soluble and thereafter isolating the compound by chromatography.
 3. A method of inhibiting a plant-pathogenic fungus which comprises applying an effective amount of the compound of claim 1 to the locus of the fungus.
 4. A composition suitable for inhibiting plant-pathogenic fungi comprising (1) an effective amount of a compound of claim 1 and (2) a suitable carrier. 